Massive Client / Server desynchronisation

I find it most when I'm walking through a tight hallway or through a doorway and it snaps me back a few blocks, thinking I hit a wall while my client didn't. I keep walking, and suddenly I can't interact with anything. Typing in the chat proves that it isn't server lag, and disconnecting and reconnecting will place me back where I was stuck. I call this position lag because of the difference in positions. It happens most often with partial blocks like stairs, chests, and doors.

I've had the same problem Eric, although for me it happens out in the open (i.e. fields etc.)

Just to be sure, is this the Problem your talking about: (see below link)

http://www.minecraftsurvivalgames.com/threads/1-3-game-ending-killing-glitch-bug.16114/

Short Description:
"All of a sudden everyone sees my player frozen but i can move but cant do anything and if i drop things they come out of my frozen player and my frozen player can attack from there but u will basically always die."

I also get the 'knockback thing'. The F3 menu and running helps prevent it though, temporarily.

It effects versions 1.3.2, and all of 1.4.

You can also see the effect in one of Etho's latest episodes.

I can confirm this will happen on a stone silver vanilla client.

Jonathan, may I ask, what do you mean by "stone silver" vanilla client?

A diamond pickaxe with Efficiency V desyncs gravel/dirt/sand, blocks disappear clientside, but the server won't let me go past the space they've occupied. Refreshing the blocks reverts them back into existence.

Minecraft 1.4.6
Oracle Java 1.7.0_10

After turns cart with me runs visually on parralel line, sometimes it rides above tracks.. after reaching certain speed strange things may happen, like cart stops while I keep moving, or after stop cart sinks into block underneath and I get stuck. Relogging results in showing expected state: cart and me are in place.
Minecraft 1.3.6 -1.4.7
Oracle Java 1.7.0_10

I thought this may be related, (i.e. its about a mod to do with this bug):

"Well, basically the way freecam works is you simply stop sending move/look updates to the server. On the client you're still moving about, but the server never gets told that so it thinks you're just standing there. So all you need to do is take the whole if/elseif block that sends out those packets, and put it inside your own if statement checking if freecam is on."

http://www.minecraftforum.net/topic/1169102-smp-free-camera-mod-help/

A really good way of reproducing:

• place a ring of Soul Sand around a Sign above a body of water as seen on the screenshot

• place a Boat in the middle of the ring by right-clicking the Sign

• enter the Boat

• you'll fall client-side into the water and can move around, while server-side you are still in the Boat around the Soul Sand

• you can leave "Ghost-Mode" by right-clicking or breaking the Boat, which will teleport you from wherever you were back to the Soul Sand structure

Yes, excellent reproduction Mod Tails, i've tested it, and it work every time. Although I had some trouble geting the boat on the sign.

Edit:
Although it only seems to work for those with the glitch.

Here's something I also thought was related:

http://dev.bukkit.org/server-mods/nocheatplus/tickets/320-erm-some-weird-freecam-bug/

Still in 1.6.2 with Tails' Soul-Sand-Ring-Boat procedure.

I've spent some time recently trying to figure out the cause of server client glitches like that. While I can't explain the scenario in the report, I think I do know what's going on with boats and why they are so bugged. There's lots of issues with boats and client/server synchronization in general, so I'll try sticking to the causes of the soul sand setup glitches, which by the way also works with just a single solid block on which you place the boat (make sure there's no adjacent blocks).

First of all, you should know there's an evil flag in the boat class. When set to false, the client takes over the control of the boat on his side. The flag is more than likely meant to be false whenever the player is inside the boat. In MCP it's called "field_70279_a". Think of it like "not ridden by local player" or "give control to server".

1) Boats glitching into the ground
The cause of this is that when the client accepts server updates (i.e. flag is true), it skips collision detection entirely, and just warps the boat to the received location. This would be fine if the positions sent by the server were always correct, but they generally are not due to optimization and bugs. One consequence of this is that when you place a boat on a full block, it will be slightly inside the ground. You don't really notice that because the client doesn't even apply gravity to the boat, at least until you get in and make the client take control of the boat, at which point the collision of the boat with the ground block goes unnoticed (because it's already inside the block), but only by the client.

2) Server/Client split
As pointed out above, once the player enters the boat, and the flag is set to false, the client entirely ignores updates from the server for the boat (without having checked that, I assume when riding something the player's position is synchronized via the vehicle, not the player itself), only the movement commands are now being synchronized, so you still move server-side.
However, there's some indication in the code that the player ridden boat is still supposed to accept position updates, but only if the discrepancy is more than 1 block. On the other hand, they could have changed it in hindsight because the movement might have ended up being jerky if the latency was high. So I guess this is either an unintended error, or the (possible unintentional) consequence of another bugfix.

3) Ghost boats
Once you leave the desynced boat, you'll notice that you (the player) returns to the correct server-side position while the boat is still off. That seems to be because of an error (in NetClientHandler) which causes the flag to stay false, even after the player leaves the boat. So any boat you enter and leave will essentially become a ghost, because the client starts ignoring updates from the server and does his own thing. Apart from restarting the game / reloading the entity, this false state is only corrected when another entity enters the boat.

I also made a mod to fix related issues: https://github.com/taurose/Unglitch . At the moment, it only fixes 1) mentioned here. I might do some more tests to figure out a clean way to fix the other issues as well though.

Is this still a concern in the latest Minecraft version 13w42b? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

Still observed in 1.7.2 SMP boats. Big problem for our ocean spawn server.

I fell into the void yesterday riding a boat in v1.7.4 (client downloaded 4-dec-13, and server set up ~14-dec-13. As a matter of client-server protocol, the client should always know if it has failed to get a "chunk" and refuse to act upon NUL data. It should never drop a player into the void because "the world disappears", and such a drop should never then be accepted by the server.

Same issue in 1.7.4 SSP. This issue makes travel by boat impossible. I can sail for perhaps 3-4 chunks before new chunks on the horizon stop loading. If I travel to this world edge and step out of the boat, I am returned to within 1-2 chunks of where I started, and my boat is another chunk or two out to sea. Evidently the game doesn't recognize I am moving, and ceases to update my location or load new chunks.

For emphasis: this is game-breaking, as I can never leave an island, or explore another continent.

I've never had this issue, I boat across oceans on my server all the time without issues (1.7.4)

Boat desync still in 1.7.4, nearly 18 months of seeing this and its still not possible to use boats.

What do you need to know to fix this?